A general configuration of a head employed in ink jet recording includes plural discharge ports, ink flow paths communicated with such discharge ports, and plural electrothermal converting elements for generating thermal energy to be utilized for ink discharge. The electrothermal converting elements are constituted by heat-generating resistors and electrodes for supplying electric power to the heat-generating resistors, and such electrothermal converting elements are covered by an insulation film to secure insulation among the electrothermal converting elements. Each ink flow path communicates, at an end opposite to the discharge port, with a common liquid chamber which stores an ink supplied from an ink tank as an ink reservoir. The ink supplied to the common liquid chamber is guided to each ink flow path, and is retained by forming a meniscus in the vicinity of the discharge port. In this state, the electrothermal converting element is selectively driven to generate thermal energy, which is utilized for causing a rapid bubbling of the ink on a heat acting surface, whereby the ink is discharged by a pressure resulting from such state change.
The heat acting portion of the ink jet head in such ink discharge is exposed to a high temperature generated by heating of the heat-generating resistor, and is also subjected mainly to a composite action of an impact of a cavitation resulting from bubble formation and contraction of the ink, and a chemical action by the ink.
Therefore, the heat acting portion is usually provided with an upper protective layer for protecting the electrothermal converting element from such impact by cavitation and such chemical action of the ink.
Conventionally, a Ta film, relatively strong against the impact by cavitation and the chemical action of the ink, is formed with a thickness of 0.2 to 0.5 μm for realizing a service life and a reliability of the head at the same time.
Also in such heat acting portion, there results a phenomenon that a coloring material or an additive substance contained in the ink is decomposed in a molecular level by heating to a high temperature to form a difficultly soluble substance which is physically adsorbed on the upper protective layer. This phenomenon is called kogation. Such adsorption of the difficultly soluble organic or inorganic substance on the upper protective layer causes an uneven heat conduction from the heat-generating resistor to the ink, thereby resulting in an unstable bubble generation. Therefore an excellent Ta film, relatively free from kogation, is employed ordinarily.
In the following, a mode of generation and extinction of a bubble in the ink in the heat action part will be explained with reference to FIG. 8.
In FIG. 8, a curve (a) indicates a change in time of a surface temperature of the upper protective layer, from a moment of application of a voltage to the heat-generating resistor, with a driving voltage Vop=1.3×Vth (Vth being a threshold voltage for bubble generation of the ink), a driving frequency of 6 kHz and a pulse width of 5 μs. Also a curve (b) indicates a growth state of a bubble generated from a moment of a voltage application to the heat-generating resistor. As indicated by the curve (a), a temperature rise starts from the voltage application, then a temperature peak is reached with a certain delay from a predetermined pulse time (because the heat from the heat-generating resistor arrives at the upper protective layer with a delay), and the temperature is lowered thereafter mainly by a heat diffusion. On the other hand, as shown by the curve (b), a bubble starts to grow at a temperature of the upper protective layer of about 300° C., then reaches a maximum bubble state and vanishes. In an actual head, this process is executed in a repeated manner. With a bubble generation in the ink, the surface of the upper protective layer rises for example to about 600° C., and this indicates how a thermal action of a high temperature is involved in the ink jet recording.
Consequently, the upper protective layer maintained in contact with the ink is required to have excellent film properties in heat resistance, mechanical properties, chemical stability, oxidation resistance, alkali resistance etc. For the material usable for such upper protective layer, in addition to the aforementioned Ta film, there are already known a precious metal, a high-melting transition metal, an alloy thereof, and a nitride, a boride, a silicide or a carbide of such metal, or amorphous silicon. For example, Japanese Patent Application Laid-open No. 2001-105596 proposes a recording head of a long service life and a high reliability by forming an upper protective layer on a heat-generating resistor through an insulation layer, and forming the upper protective layer with an amorphous alloy represented by TaαFeβNiγCrδ (wherein 10 atomic % (at. %)≦α≦30 at. %, and α+β>80 at. %, and α<β, δ>γ and α+β+γ+δ=100 at. %) in which a surface thereof in contact with the ink includes an oxide of a constituent thereof.
However, a higher functionality such as a higher image quality and a higher recording speed for an image recorded by an ink jet recording apparatus is being required more strongly in recent years, and, in order to meet such requirement, there are desired an improvement in ink performance such as an improvement in color developing property and weather resistance for achieving a higher image quality and a prevention of a bleeding phenomenon (blotting between inks of different colors) in order to achieve a higher recording speed. For this reason, it has been tried to add various components to the ink. Also types of the ink have become diversified, such as light-colored inks of lower density in addition to black, yellow, magenta and cyan colors. Such inks cause a corroding phenomenon even on the Ta film, that has been considered stable as the upper protective film, by a thermal chemical reaction with such inks. Such phenomenon appears conspicuously in an ink containing a salt of a divalent metal such as Ca or Mg, or a component forming a chelate complex.
On the other hand, an upper protective layer with an improved corrosion resistance to the ink as explained above tends to generate a kogation more easily since the surface is scarcely damaged because of the higher corrosion resistance, whereby a discharge speed of the ink is lowered or becomes unstable. The kogation is generated little in the conventional Ta film presumably because the Ta film generates corrosion and kogation in a certain balanced level whereby the surface of the Ta film is abraded by such corrosion to suppress deposition of a product of kogation.
Also for achieving a further higher recording speed in the ink jet recording, it is necessary to further increase the drive frequency thereby executing a drive with shorter pulses. In such drive with shorter pulses, processes of heating, bubble generation, bubble extinction and cooling are repeated within a shorter period in the heat acting portion of the head, whereby a larger thermal stress is generated in a shorter time than in the conventional drive. Also in a drive with a shorter pulse, the cavitation impact resulting from the bubble generation and bubble contraction in the ink is concentrated in the upper protective film within a shorter time than in the conventional drive, whereby there is required an upper protective layer particularly excellent in the mechanical impact resistance.
For forming an ink jet head with an ink jet head substrate provided with such upper protective layer, there is employed, as disclosed in Japanese Patent Application Laid-open No. H6-286149, a method of forming an ink flow path with a soluble resin by a photolithographic patterning, then covering and hardening such pattern with an epoxy resin or the like, and eliminating such soluble resin after the substrate is cut into a piece.
It is also possible, as disclosed in Japanese Patent Application Laid-open No. 2002-113870, to achieve a higher durability and a higher reliability by constituting the upper protective layer with two layers, employing an amorphous Ta film of a high ink corrosion resistance as a lower layer and a Ta film of relatively low generation of kogation as an upper layer.
However, in case of elongating an ink discharge element (to 0.5 inches or larger) for achieving a higher recording speed or in case of employing diversified inks containing additives for improving a light fastness or a gas resistance of the inks on a recording medium, there are generated strains by a difference in the linear expansion coefficient of such components, and by a stress in the resin layer constituting walls of the liquid flow path or the discharge port, and an influence on the interface by inks of new types, thus leading to a peeling phenomenon between the covering resin layer constituting the walls of the liquid flow path or the discharge port and the upper protective layer on the heater substrate. Also, even in case an organic adhesion promoting layer is provided on the upper protective layer, there may result a peeling at the interface between the organic adhesion promoting layer and the upper protective layer to cause a penetration of the ink onto the substrate and to induce a corrosion of the wirings, thereby hindering satisfactory recording or reliability in quality over a prolonged period.